Conventional thermoforming involves heating a polymer sheet to a pliable forming temperature (which depends at least in part on the type of sheet being heated), forming the sheet to a specific shape on a mold, and thereafter trimming unformed portions of the sheet to create a useful product. The sheet, sometimes referred to as “film” when thin gauges or certain types of materials are formed, is typically heated in an oven to the forming temperature so that it may be stretched into or onto a mold and then cooled to retain a finished shape. During the heating process, the entire sheet of material is heated to the forming temperature. Portions of the sheet that are not formed are usually referred to as “trim” and not reused until after further processing.
U.S. Pat. No. 4,878,826 to Wendt, the entire contents of which are incorporated herein by this reference, is one of many patents disclosing apparatus for thermoforming articles from sheets of plastic material. The apparatus of the Wendt patent may include both male and female molds together with a heating means and evacuation equipment. One such heating means is described as being hot oil circulating through an associated manifold such that it crystallizes a sheet of plastic material. See Wendt, col. 10, 11. 9-12. According to the Wendt patent, the sheet also may be pre-heated to 10-15% crystallization before entering the mold. See id., col. 12, 11. 50-57. Indeed, over-crystallization of the sheet apparently is an issue with the apparatus of the Wendt patent, requiring cold air to be injected into various mold cavities. See id., col. 13, 11. 53-65.
Thermoforming a plastic sheet necessarily distorts it. However, in some circumstances distortion of certain portions of a sheet is undesirable. As an example, distortion of portions of a sheet containing printing or art work may render them unintelligible or, at minimum, diminish their aesthetic appeal. Consequently, conventional thermoforming requires pre-printing of text and art in a distorted form so that the further distortion caused by the thermoforming can counteract the pre-distortion and, at least theoretically, produce intelligible images. Thus, providing apparatus and methods that would allow thermoforming of selected portions of a plastic sheet while avoiding distortion of other portions of the sheet thus would be a beneficial—albeit difficult—achievement.